Apparatus for minimizing image smear due to ion caused undercutting

ABSTRACT

Apparatus for minimizing the smear of an image formed by xeroradiography. In one embodiment, a distributed capacitor, positioned adjacent to and substantially parallel to a sensitized plate and electrically connected thereto, is charged before an object is exposed to penetrating radiation, the electric field generated by the charged capacitor minimizing image smear. In a second embodiment, a metal electrode is coupled to the conductive substrate which comprises part of the sensitized plate via a lumped capacitor, the electric field generated between the metal electrode and plate minimizing image smear.

[ Dec. 18, 1973 APPARATUS FOR MINIMIZING IMAGE SMEAR DUE TO ION CAUSED UNDERCUTTING [75] Inventor: Vernon W. Dryden, Pasadena, Calif.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: July 6,1971

211 App]. No.: 159,904

52 .s. g 250 515, 11/2 2 A 51 Int. c|...;;"G0'In 21/341'H01 37/26, HOSg 1/60 581 Field 6: Search 250/65 ZE, 49.5;

Primary Examiner-James W. Lawrence Assistant Examiner-T. N. Grigsby Atmrney-James J Ralabate et al.

[57] ABSTRACT Apparatus for minimizing the smear of an image formed by xeroradiography. In one embodiment, a distributed capacitor, positioned adjacent to and substantially parallel to a sensitized plate and electrically con nected thereto, is charged before an object is exposed to penetrating radiation, the electric field generated by the charged capacitor minimizing image smear. In a second embodiment, a metal electrode is coupled to the conductive substrate which comprises part of the sensitized plate via a lumped capacitor, the electric [56] R fer Cit d field generated between the metal electrode and plate PATENTS minimizing image smear.

2,802,948 8/1957 Vyverherg 250/65 ZE 7 Claims, 6 Drawing Figures PAIENIEUMB 18 ms 3. 780.288

SHEEI 1 or 2 vFIG. 16 FIG. 1d 27 FIG 2 I INVENTOR VERNON W. DRYDEN ATTORNEY l APPARATUS FOR MINIMIZING IMAGE SMEAR DUE TO ION CAUSED UNDERCUTTING BACKGROUND OF THE INVENTION Image smear, due to ion-caused undercutting, is a non-faithful representation of the inner structure of a test object manifested as a loss of detail in image areas of a xeroradiographic reproduction. Image smear occurs primarily in those low contrast regions of the image which receive the largest amount of ray energy. Undercutting occurs at an edge between regions of high and low X-ray exposure.v

It is well known that the longer wavelength X-rays are efficient in ionizing the air near the surface of a xerographic photo-emitting receptor during exposure. The amount of air ionization is proportional to the intensity of the X-rays reaching the photoreceptor surface. The mechanism of ionization of the air includes the direct photoelectric action of both X-rays photons on the air and the secondary photoelectrons from the photoreceptor on the adjacent air molecules. Both positive and negative ions are formed in the process. If these ions are allowed to diffuse randomly, they will partially discharge the electrostatic image on the photoreceptor in a manner which tends to destroy rather than enhance the latent image of the object being examined. xerographic undercutting is caused by the ionization of air above the surface of the xeroradiographic plate. In particular, during exposure to a specimen of differing thicknesses, the xerographic plate is discharged more completely at the thinner sections, causing discontinuities in the charge pattern. Across these discontinuities, strong localized fields curve sharply from one charge concentration to the other. The ions having a polarity opposite to the charge on the photoreceptor are attracted by the local fields, and are deposited on and discharge the more highly charged sides of voltage discontinuities. As the exposure continues, and the edges become discharged by the action of the ions, the field pattern moves inward toward the center of the more highly charged area; additional ions will follow, destroying more and more of the image.

The technique of minimizing ion undercutting in xeroradiography by positioning a biased electrode in spaced proximity to the photoreceptor surface is shown by Vyverberg U. S. Pat. No. 2,802,948. As set forth therein, the electrode is biased in the range from 700 to 1,800 volts, preferably 1,200 volts,'by a D.C. voltage source. v

In the xeroradiographic system described in copending application Ser. No. 874,834, filed on Nov. 7, 1969, now US. Pat. No. 3,650,620, a photoreceptor is automatically charged and inserted into a cassette unit. The cassette unit, forming a light-tight environment for the enclosed photoreceptor, may be transported to an exposure station whereat the object to be examined is exposed to X-rays. To minimize image smear, the system disclosed by Vyverberg requires the aforementioned voltage source to generate an appropriate electric field whereby negative ions are prevented from discharging the latent electrostatic image. The portability of the exposure system disclosed in Vyverberg is therefore severely limited by the weight and bulkiness of the required voltage source.

SUMMARY OF THE INVENTION The present invention provides means for minimizing image smear, due to ion undercutting, in xeroradiography. In particular, a charged photoreceptor is enclosed in a light-tight cassette unit. In the preferred embodiment, a distributed capacitor, comprising an insulating layer sandwiched between conducting layers, is formed in the lid of the cassette unit. The photoreceptor, comprising a photoconductor overlying a conductive substrate, is supported adjacent the cassette bottom. The distributed capacitor is initially charged by a voltage source which is then removed. The object to be examined is exposed to X-rays and a latent electrostatic image is formed on the photoconductor surface. The distributed capacitor retains the charge initially applied thereto for a substantial time period, thereby enabling a plurality of exposures to be made with minimization of image smear without recharging the capacitor. In an alternate embodiment, a metal electrode is formed in the lid of the cassette and coupled to the conductive substrate via a lumped capacitor, the lumped capacitor being initially charged as described in reference to the distributed capacitor.

It is an object of the present invention to provide improved means for minimizing image smear in xeroradiography.

It is a further object of the present invention to minimize image smear in a xeroradiographic system wherein the charged photoreceptor is enclosed in a light-tight cassette unit.

It is still a further object of the present invention to provide apparatus for minimizing image smear in xeroradiography wherein a conducting electrode is positioned adjacent the photoreceptor surface and coupled to the photoreceptor substrate via a lumped capacitor, the field produced by the capacitor being sufficient to minimize image smear.-

It is a further object of the present invention to provide apparatus for minimizing image smear in xeroradiography wherein a charged distributed capacitor is positioned adjacent the photoreceptor surface, the electric field produced by said'capacitor being sufficient to minimize image smear.

DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I a there is shown a schematical representation of one embodiment of the present invention. The xeroradiographic plate or photoreceptor 10 comprises generally a layer of an insulating photoconductor 12, such as vitreous selenium, overlying a conductive metal backing plate 14. As is well known in the xerographic art, layer 12 has the property of retaining charge in the dark and of discharging or dissipating its retained charge when exposed to light, X-rays, gamma rays or other penetrating radiation.

Spaced apart from, for example, within 1 cm, and substantially parallel to plate 10, is element 16 com prising a thin layer of dielectric material 18 sandwiched between two X-ray transparent, electrically conductive surfaces 20 and 22. Typical examples of conductive material include aluminum, copper, brass, etc. Typical dielectric materials include Mylar, comprising po]yeth ylene teraphthalate and Teflon, comprising tetrafluoroethylene, both trademarks of E. I. Dupont de Nemours, Inc. Conducting surface 20 is electrically connected to metal backing plate 14 via resistor 24. An electric field perpendicular to the surface of the photoconductor 12 is generated in the region between the xerographic plate and conducting surface 22 due to the initial charge which is utilized to sensitize the surface of photoconductor 12. The electric field in this region is further enhanced by placing an electrostatic charge on conducting surfaces and 22. Dielectric material 18 and conducting surfaces 20 and 22 form a distributed capacitor which is charged by connecting voltage source 28 across the conducting surfaces. Voltage source 28 is illustrated in a manner to indicate that after it charges conducting surfaces 20 and 22, it is removed from contacts 25 and 27 prior to the exposure of object 26 by the X-ray source 29.

The thickness of dielectric layer 18, when formed of Mylar, is about 0.003 inches whereas the conducting surfaces 20 and 22, when formed of aluminum, are approximately 0.0005 inches thick. The value of resistor 24 is typically 10 ohms. In operation, DC. voltage source 28 is connected across conducting plates 20 and 22 through resistor 24 and applies a potential thereacross in the range from about 400 volts to about l,600 volts, depending on the initial charge placed on photo conductive layer 12, and of a polarity as shown that a negative charge is formed on conducting surface 22. It is assumed that the surface of photoconductor 12 has initially been charged to a positive potential. Object 26 is then exposed to penetrating radiation generated by X-ray source 29. The voltage source 28 may then be disconnected from conducting plates 20 and 22 or, alternatively, may be disconnected prior to exposure.

The electric field perpendicular to the surface of photoconductor 12 tends to confine the air ions to the immediate region where they are generated. Ions charged to the same polarity as the polarity of the electrostatic charge on the photoconductor surface are at tracted to the conducting surface 22 while the ions charged to a polarity opposite to that of the electrostatic charge on the surface of the photoconductor tend to further discharge the photoconductor 12 in the same pattern as the X-rays which expose the photoconductor 12. Consequently, the image of the object being examined is improved due to the presence of the electric field generated by the capacitor 16 which is maintained even though the plate is discharged by the exposure. The electric field tends to maintain ions in the position over the plate where they were formed (unless they have excess lateral energy from their formation). Negative ions are forced toward the plate contributing constructively to the exposure while positive ions are forced away from the plate. The distributed (sheet) capacitor 16 does not need a permanent charging source and may be charged, as set forth hereinabove, just before exposure. Experience with modest energy X-ray (in the range from about 25 KVP to about 45 KVP) exposures shows that capacitor 16 will maintain its charge for at least exposures.

It should be noted that if photoconductor layer 12 is initially negatively charged, the charge applied to conducting surfaces 20 and 22 should be reversed by reversing the connections of voltage source 28, i.e., a positive charge is formed on surface 22. In this manner, minimization of image smear is also accomplished regardless of the polarity of the initial charge on the surface of photoconductor 12.

Since capacitor 16 is chosen to be transparent to X-ray radiation, the X-ray radiation does not effect the charge thereon.

In practice, it has been found that the sum of the charge on photoconductive layer 12 and the charge placed on the distributed capacitor 16 should be in the range of about 400 volts to about 2,000 volts. The plates are normally charged in the range from about 400 volts to about 1,600 volts. For illustrative purposes, if the plate is charged to 400 volts, the potential applied to the capacitor may be l,600 volts.

Resistor 24 is not necessary for the operation of the present invention. However, resistor 24 acts to prevent the thin aluminum surfaces from burning off the electrodes 20 and 22 around the contact area (illustrated in FIGS. 2-5 hereinbelow) during charging of capacitor 16 or if conducting surface 22 is accidentally brought into contact with photoconductive layer 12. In addition, resistor 24 protects against shock hazard when used, for example, in the cassette illustrated in FIGS. 2-5. In lieu of resistor 24, substrate 14 may be connected directly to conducting surface 20 or via a resistor of smaller resistance value.

FIG. 1 b illustrates an alternate embodiment which utilizes a lumped capacitor 16 to couple metal electrode 22' to substrate 14 via resistor 24. The operation of the invention described in this embodiment is identical to that described with reference to FIG. 1a. The physical size of capacitor 16 required to produce the desired results makes the embodiment of FIG. 1a, which utilizes distributed capacitor 16, the preferred embodiment for the purposes of the present invention.

FIGS. 2-5 show views of the embodiment shown in FIG. 1a when utilized in a cassette unit of the type described in copending application U. S. Ser. No. 874, 747 filed Nov. 7, 1969, now abandoned. The details of the cassette shown in the copending application are not set forth herein since the inventive concept of the present invention set forth hereinabove may be utilized in any X-ray imaging cassette. Therefore, the figures shown are highly schematical views of the present invention when utilized in combination with a cassette.

FIG. 2 is a plan view of a cassette unit 30, in the closed position, shown in a transparent form to illustrate the capacitor 16 clearly. Conducting surface 20 is connected to a 10 ohms deposited resistor 24 (a lumped resistor may be utilized instead) via conductor 32. Conductor 32 may comprise a conducting tape formed of aluminum with a conductive adhesive, a deposited aluminum film, copper tape, etc.

FIG. 3 is a front elevation of the cassette 30 showing the substrate of photoreceptor plate 10 connected to conducting surface 20 via conductor spring 34, spring contact 36 and deposited resistor 24. Also shown are guide rails 35 for supporting the photoreceptor plate within the cassette.

FIG. 4 is a side elevation of cassette 30 further illustrating the connection of conducting surface 20 to the substrate 14 of plate via conductor spring 34.

FIG. 5 is a plan view of conducting surface 22 with contact surface 38 for conductor spring 34. Contact 25 is connected to conducting surface 22 via a strip of conducting tape 29 as shown. Contact 27 is connected to contact surface 38 via thin conducting strip 31.

The distributed capacitor 16 is placed inside the eassette lid and held thereto by a suitable adhesive with the top conducting surface electrically connected to the metallic substrate of plate 10 as shown.

The present invention provides an additional advantage to those discussed hereinabove. 1f the cassette material is made of plastic, or other insulating material, a surface electrical charge may be formed in various patterns on different portions of the cassette. This will lead to a non-uniform electric field above the surface of photoreceptor H) which is usually detrimental to the image to be formed and reduces the possibility of image repeatability. The utilization of the distributed capacitor 16 in the manner described hereinabove substantially eliminates this problem by providing a uniform electric field of a magnitude which masks the effect of the non-uniform fields.

While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.

What is claimed is:

1. Apparatus for minimizing image smear of an image formed by exposing an object to penetrating radiation, the penetrating radiation transmitted by said object forming a latent electrostatic image on the surface of an initially charged photoconductor layer positioned adjacent thereto, said photoconductor layer overlying a conductive substrate comprising:

a distributed capacitor spaced apart from and substantially parallel to the surface of said photoconductor, said distributed capacitor comprising an insulating layer sandwiched between layers of conductive material,

means for electrically connecting one layer of said conductive material to said conductive substrate,

' means for charging said capacitor to a predetermined potential prior to exposing said object to said penetrating radiation, and

means for exposing said object to said penetrating radiation whereby a latent electrostatic charge pattern corresponding to the intensity of the radiation transmitted through the object is formed on said photoconductor layer.

2. The apparatus as defined in claimv 1 wherein the charge formed on the layer of conductive material fac ing said photoconductor layer is opposite in polarity to the polarity of the charge initially formed on said photoconductor surface.

3. The apparatus as defined in claim 1 wherein said charging means is removed prior to exposing said object to said penetrating radiation.

4. A cassette for enclosing in a light-tight environment a photoconductor layer formed on a conductive substrate, said photoconductor layer having a latent electrostatic charge formed thereon, said cassette including lid and bottom portions, said photoconductor layer being supported in the bottom portion comprismg:

a distributed capacitor spaced apart from and substantially parallel to the surface of said photoconductor layer, said distributed capacitor comprising an insulating layer sandwiched between layers of conductive material and attached to the inside of said lid, and

means for electrically connecting one layer of said conductive material to said conductive substrate.

5. The cassette as defined in claim 4 further including means within said cassette for electrically coupling a source of potential applied thereto across said layers of conductive material.

6. A cassette having lid and bottom portions comprisa distributed capacitor supported within said lid portion and comprising an insulating layer sandwiched between layers of conductive material,

means for electrically, coupling a source of potential applied thereto across said layers of conductive material,

means in said cassette bottom portion for supporting a plate member inserted in said cassette, said plate member spaced apart from and substantially parallel to said distributed capacitor, and

means for electrically connecting one layer of said conductive material to said plate member.

7. The cassette as defined in claim 6 wherein said plate member comprises a photoconductor layer overlying a conductive substrate, said layer of conductive material being electrically connected to said conductive substrate. 

1. Apparatus for minimizing image smear of an image formed by exposing an object to penetrating radiation, the penetrating radiation transmitted by said object forming a latent electrostatic image on the surface of an initially charged photoconductor layer positioned adjacent thereto, said photoconductor layer overlying a conductive substrate comprising: a distributed capacitor spaced apart from and substantially parallel to the surface of said photoconductor, said distributed capacitor comprising an insulating layer sandwiched between layers of conductive material, means for electrically connecting one layer of said conductive material to said conductive substrate, means for charging said capacitor to a predetermined potential prior to exposing said object to said penetrating radiation, and means for exposing said object to said penetrating radiation whereby a latent electrostatic charge pattern corresponding to the intensity of the radiation transmitted through the object is formed on said photoconductor layer.
 2. The apparatus as defined in claim 1 wherein the charge formed on the layer of conductive material facing said photoconductor layer is opposite in polarity to the polarity of the charge initially formed on said photoconductor surface.
 3. The apparatus as defined in claim 1 wherein said charging means is removed prior to exposing said object to said penetrating radiation.
 4. A cassette for enclosing in a light-tight environment a photoconductor layer formed on a conductive substrate, said photoconductor layer having a latent electrostatic charge formed thereon, said cassette including lid and bottom portions, said photoconductor layer being supported in the bottom portion comprising: a distributed capacitor spaced apart from and substantially parallel to the surface of said photoconductor layer, said distributed capacitor comprising an insulating layer sandwiched between layers of conductive material and attached to the inside of said lid, and means for electrically connecting one layer of said conductive material to said conductive substrate.
 5. The cassette as defined in claim 4 further including means within said cassette for electrically coupling a source of potential applied thereto across said layers of conductive material.
 6. A cassette having lid and bottom portions comprising: a distributed capacitor supported within said lid portion and comprising an insulating layer sandwiched between layers of conductive material, means for electrically coupling a source of potential applied thereto across said layers of conductive material, means in said cassette bottom portion for supporting a plate member inserted in said cassette, said plate member spaced apart from and substantially parallel to said distributed capacitor, and means for electrically connecting one layer of said conductive material to said plate member.
 7. The cassette as defined in claim 6 wherein said plate member comprises a photoconductor layer overlying a conductive substrate, said layer of conductive material being electrically connected to said conductive substrate. 